docs: def Planck_wavenumber(n, T):
      '''The Planck function (flux density for blackbody radition)
      in wavenumber space
      n is wavenumber in 1/cm
      T is temperature in Kelvin
  
      Formula from Raymond Pierrehumbert, "Principles of Planetary Climate", page 140.
      '''
      # convert to mks units
      n = n*100.
      return c_light * Planck_frequency(n*c_light, T)